Semiconductor pressure transducers are frequently used in applications which require operation in harsh environments at high temperatures. These environments are corrosive and/or involve high temperatures. Thus, many transducers used in such applications are protected from these harsh environmental conditions in order for the transducer to remain operational over extended periods of time at elevated temperatures.
The Assignee herein, Kulite Semiconductor Products, Inc. has a number of patents which show transducers which are operational at extremely high temperatures in extremely harsh environments. A particular problem which is inherent in such transducers involves the contacts and output leads of the transducer as the contacts and output leads could fail at temperatures of 600° C. or greater. While these temperatures are very high it is, of course, desirable to operate at even higher temperatures.
In the prior art, a slice or wafer of silicon from which the sensor is formed had appropriate high temperature platinum based contact metallization and was bonded to a glass wafer. The glass wafer was typically Pyrex and the structure enabled one to provide an extremely high temperature transducer. The Pyrex had apertures which extended to the platinum based contacts associated with the silicon sensor and positioned on the silicon slice or wafer.
Contact was glass made to the platinum region of the silicon sensor wafer by means of a metallic frit and contact was made to the frit via very short pins on a header.
For examples of such devices, reference is made to Kulite U.S. Pat. No. 5,955,771 (771) entitled “SENSORS FOR USE IN HIGH VIBRATIONAL APPLICATIONS AND METHODS OF FABRICATING THE SAME”, issued on Sep. 21, 1999 to A. D. Kurtz et al. and assigned to the Assignee herein. This patent shows a semiconductor chip which is hermetically bonded and sealed to the mounting surface of a Pyrex glass cover member.
The semiconductor chip has one or more contacts disposed on the surface for making electrical contact thereto. The Pyrex cover has one or more contact apertures extending therethrough which exposes a portion of the contacts. The portion of each pin extending above the mounting surface is received within the contact aperture and a conductive glass frit mixture is disposed in the contact apertures. The glass frit hermetically seals the contact apertures and provides electrical continuity between the pins and the contacts. See, for example, column 6, lines 44 to 67 of the 771 patent.
In any event, this prior technique operates quite well in harsh environments at temperatures over 500° C. and greater. See also, U.S. Pat. No. 5,973,590 (590) entitled “ULTRA THIN SURFACE MOUNT WAFER SENSOR STRUCTURES AND METHODS FOR FABARICATING THE SAME”, issued on Oct. 26, 1999 to A. D. Kurtz et al. and assigned to the Assignee herein.
Referring to FIG. 10 of the 590 patent, a glass wafer is bonded to a silicon wafer. The glass wafer is electrostatically bonded to the silicon wafer and has apertures which are filled almost to the top surface with an unfired metal glass frit. Small spheres or balls of metal are inserted into the unfired frit leaving a portion of the ball protruding past the top edge. This provides a contact for the entire structure. In this the head portions of the header pins can be attached or further secured to the semiconductor surface by means of a gold epoxy or other adhesive substance to securely fasten the pins within the structure.
Reference is also made to U.S. Pat. No. 6,058,782 (782) entitled “HERMITICALLY SEALED ULTRA HIGH TEMPERATURE SILICON CARBIDE PRESSURE TRANSDUCERS AND METHODS FOR FABRICATING THE SAME”. This patent issued on May 9, 2000 to A. D. Kurtz et al. and assigned to the Assignee herein. As seen this patent differs from the above noted patents in the fact that high temperature transducers using silicon carbide are employed. Even though these transducers are capable of operation at higher temperatures than silicon structures, the leads still had to be bonded to the substrate.
As one can ascertain, there is shown a second substrate of silicon carbide having a plurality of apertures extending therethrough, which apertures align and correspond to an associated contact on the contact area of the sensor substrate and is joined to that substrate by electrostatic bonding or by employing a glass frit. The apertures are filled with a glass metal frit mixture which includes platinum to provide a hermetic seal of the sensor element. A gold plated spherical contact pin is inserted into the glass metal frit filled apertures to provide contact means for the transducer.
An example of such a transducer in cross sectional view is shown in FIG. 4 of the 782 patent. As can be seen, the device depicted in the 782 patent is essentially a lead less device. The pins are coplanar with the second substrate and protrude into it. The pins are fabricated from nickel or Kovar which is a high temperature steel alloy. Pins are inserted into the filled apertures of the second substrate. The apertures are filled with a metal glass frit as indicated above. Such transducers as fabricated from the techniques described in the 782 patent can operate at temperatures of 600° C. or greater over an extended period of time.
Reference is also made to U.S. Pat. No. 6,210,989, (989) issued on Apr. 3, 2001 and entitled “ULTRA THIN SURFACE MOUNT WAFER SENSOR STRUCTURES AND METHODS FOR FABRICATING SAME”. This patent issued to A. D. Kurtz et al. and is assigned to the Assignee herein. The 989 patent is a divisional of U.S. Pat. No. 5,973,590 as described above and essentially shows the apertures aligned with the first glass member and having a group of hermetically sealed pins for coupling to the contact locations.
In regard to the above noted techniques, the high temperature platinum based metallization contacts are on the sensor wafer. The sensor wafer is associated with and bonded to a glass wafer having contact apertures. The apertures extended to the platinum based contacts on the silicon sensor wafer. Contact was made to the platinum region of the silicon sensor wafer by means of a conductive metallic frit and contact was made to the frit by very short pins on a header.
In any event, in spite of the vast improvement in temperature operation it has been discovered that the upper temperature limit of the bond or the interconnection between the platinum metallization on the silicon sensor and the conductive glass metal frit was in the neighborhood of 625° C. While this temperature is extremely high, it has been discovered that a substantial improvement can be achieved.
It is therefore an object of the present invention to provide improved contact or interconnect structures which enable operation of pressure transducers to extend to well over 700° C. Thus, it is really intended to produce high temperature leads or interconnect for any type of semiconductor wafer especially wafers which include piezoresistor devices for high temperature transducer operation.